GENERAL PHYSICAL CONSTITUTION OF LIVING MATTER 39 



further action of the two colloids upon each other. He called this film 

 the membrane of precipitation.* The following example may be quoted : 

 "A solution of a certain gelatine -- /3-gelatine -- was prepared, and a 

 drop taken out of this solution with a glass rod. The drop remained 

 hanging at the end of the rod, and was exposed to the air for several 

 hours; it was then dipped in a 5 per cent solution of tannic acid. In 

 about ten minutes a thin iridescent solid film formed at the surface of 

 the drop. The /3-gelatine and the tannic acid had formed a membrane 

 of precipitation at the common surface, which was impermeable for 

 both colloids and thus prevented any further reaction between the two." 

 But it is not necessary for the formation of membranes of precipitation 

 that two colloids act upon each other. A crystalloid and a colloid may 

 form a membrane of precipitation, as is the case when a drop of tannic 

 acid is dipped into a neutral solution of lead acetate. Two crystal- 

 loids can also form such membranes if they only form an amorphous 

 precipitate which is impermeable for both crystalloids, e.g. ferrocyanide 

 of potassium and ferric chloride. While these membranes are imper- 

 meable for certain substances, they are not so for others; and Traube 

 recognized the fundamental importance of this fact for life phenomena, 

 "The cell membrane f makes a diminutive chemical factory of the con- 

 tents of this cell by shutting it off from its surroundings, and enables 

 each cell to lead a specifically different life from the neighboring cells." 

 The substances which can permeate the membranes of precipitation 

 vary according to the nature of the latter. All of them allow water to 

 pass through; while they do not allow sugar or salts to pass through 

 at all or not equally well. Traube pointed out that this semipermea- 

 bility also explains the mechanism of cell growth. When the drop of 

 /3-gelatine (or any other substance used for the experiment) had a 

 greater concentration than the solution into which it was dipped, the 

 drop began to grow in size as soon as the membrane of precipitation 

 was formed. Traube thus became the originator of the modern theory 

 of the growth of cells, which assumes that the growth is caused by the 

 cell absorbing water in consequence of its osmotic pressure being higher 

 than that of the surrounding solution. 



Traube was inclined to explain the semipermeability of his artifi- 

 cial membranes on the basis of the assumption that they possess very 

 small pores or interstices which allowed only small molecules, such as 

 water, to permeate; while the larger molecules, such as salts, could 

 not pass through them. This assumption was no longer tenable after 



* M. Traube, Rdcherfs und Du Bois Reymond's Archiv, 1867. Gesammdte Abhand- 

 htngen, p. 213, Berlin, 1899. 



t We should now say, the surface film of protoplasm. 



